The proto-oncogene encoding the Tpl2 kinase is activated by provirus integration in retrovirus-induced rodent lymphomas and mammary adenocarcinomas. Here, we present evidence that Tpl2 also functions as a tumor suppressor gene in the intestinal epithelia. To determine the role of Tpl2 in Apcmin-induced tumorigenesis, we crossed the Apcmin mutation into the Tpl2-/- genetic background. Contrary to expectation, Tpl2-/-/ Apcmin/+ mice exhibited a 5-fold increase in the number of intestinal polyps and a significant acceleration in tumorigenesis relative to their Tpl2+/+/Apcmin/+ littermates. The high number of polyps in Tpl2-/-/ Apcmin/+, relative to Tpl2+/+/Apcmin/+ mice suggested that Tpl2 ablation promotes tumor initiation. Since the earliest genetic change responsible for tumor initiation is the loss of heterozygocity (LOH) in the APC locus, it follows that Tpl2 ablation promotes LOH. This could be caused by several mechanisms. One possibility is that Tpl2 ablation increases the number of intestinal stem cells that may be the targets of LOH. Given that the Wnt pathway promotes the cycling of stem cells, we hypothesized that Tpl2 ablation may cause an increase in stem cell numbers, by stimulating Wnt signaling. Studies presented here however showed that, although Tpl2 has the potential to inhibit Wnt signaling in several types of epithelial cells, Tpl2 ablation does not increase the number of stem cells in the intestinal crypts. Further studies revealed that Tpl2 ablation promotes intestinal inflammation, which is also known to stimulate tumor initiation. A search for the causes of inflammation revealed that Tpl2 ablation causes a Paneth cell defect, characterized by significant downregulation of genes encoding antimicrobial peptides. The same search revealed that inhibition of TLR signals that may be transduced via Tpl2, stimulates the activity of the canonical Wnt pathway in macrophages and that the levels of IL-10 in the intestinal mucosa of Tpl2-/- mice are decreased. The role of Tpl2 however, may not be restricted to tumor initiation. Our preliminary data indeed provide strong evidence that Tpl2 ablation may also contribute to the late stages of oncogenesis. Tpl2 ablation may modulate the biology of tumor cells induced by the ablation of APC by targeting the Wnt pathway, or perhaps other interacting pathways. Alternatively, it may target Akt and mTOR, as suggested by our recent studies on human colon carcinoma cell lines. Work proposed in this application will focus on the role of Tpl2 in the regulation of pre-LOH and post-LOH events in intestinal tumorigenesis induced by APC mutations.